This invention generally relates to needle retraction mechanisms. More particularly, this invention relates to needle retraction mechanisms that are used in the medical field. Such needle retraction mechanisms find particular applicability in connection with intravascular catheters, syringes, blood collection tubes and lancets.
Catheters, particularly intravenous (IV) catheters, are used for infusing fluid, such as normal saline solution, various medicaments and total parenteral nutrition, into a patient or withdrawing blood from a patient. Peripheral IV catheters tend to be relatively short, and are on the order of about one and one-half inches in length. The most common type of IV catheter is an over the needle peripheral IV catheter. As its name implies, an over the needle catheter is mounted over an introducer needle having a sharp distal tip. The catheter and the introducer needle are assembled so that the sharp distal tip of the introducer needle extends beyond the distal tip of the catheter with the bevel of the needle facing up away from the patient's skin.
The catheter and introducer needle assembly is inserted at a shallow angle through the patient's skin into a peripheral blood vessel, i.e a smaller blood vessel that is not connected directly to the heart but is one of the branches of the central blood vessels that is directly connected to the heart. In order to verify proper placement of the assembly in the blood vessel, the clinician confirms that there is flashback of blood in the needle and in a flashback chamber located at the proximal end of the needle in conjunction with the needle hub. Once proper placement is confirmed, the clinician applies pressure to the blood vessel by pressing down on the patient's skin distal to the tip of the needle and the catheter. This finger pressure minimizes further blood flow through the catheter and needle. The clinician advances the catheter into the blood vessel, withdraws the needle, leaving the catheter in place, and attaches a fluid handling device to the catheter hub.
Once the introducer needle is withdrawn from the catheter, it is a "blood contaminated sharp" and must be properly handled. With the recognition by the medical device industry of the risk of transmission of Acquired Immunosuppressive Deficiency Syndrome (AIDS) by blood contaminated sharps, various needle shielding mechanisms have been developed. One type of a needle shielding mechanism uses a substantially hollow handle with an introducer needle movably disposed in the handle. In such a device, the sharp distal tip of the needle may be extended from a hollow handle so the sharp distal tip of the needle is exposed. After the needle has been used to place a catheter into a patient, the needle can be retracted into the handle so that the sharp distal tip of the needle is no longer exposed. Various biasing mechanisms can be used to allow the introducer needle to be retracted into the handle after use. For example, a helical spring, either in compression or tension, could be used to provide the biasing force. Alternatively, an elastic tube could be used to provide the biasing force. In order to minimize the number of parts needed for the device, a vacuum created in the proximal portion of the handle between the proximal wall of the handle and the proximal portion of the needle/needle hub can be used to provide the biasing force to retract the needle into the handle.
Although such devices generally work for their intended purpose, they could be improved. For example, it is desirable that the needle be retracted into the handle with a specific speed profile. For example, the retraction speed should not be too slow. If it is too slow the sharp distal tip of the needle will be exposed longer than is necessary after the procedure. Increased exposure time increases the risk of an accidental needle stick. On the other hand, the retraction speed should not be too fast. If the retraction speed is too fast, any blood in the needle, flashback chamber or on the exterior of the needle could become airborne during needle retraction and could be deposited outside of the handle. Such a potential blood splatter issue is especially problematic when the needle is initially removed from the catheter after the catheter has been properly placed in the patient because significant amounts of blood may have contaminated the device. This is unacceptable when the blood could be contaminated by blood borne pathogens.
Where a helical spring or an elastic tube is used as the biasing mechanism, the spring or tube can be designed to ensure that the needle is retracted with an appropriate speed within a narrow range of values. However, where a vacuum is used as the biasing mechanism, control of the retraction speed becomes problematic since the biasing mechanism is atmospheric pressure. Because the atmospheric pressure varies with altitude, the needle retraction speed varies significantly when the device is used at a location that is at sea level such as Los Angeles and New York City and when the device is used a locations that are at high altitudes such as Salt Lake City, Utah and Mexico City, Mexico. At lower elevations, the atmospheric pressure has a certain value which is greater than the atmospheric pressure at higher elevations. This results in greater force being applied against the vacuum when the device is used at lower elevations than when the device is used at higher elevations. This means that if the device is designed for use at lower elevations, the device will tend to retract the needle significantly slower when the device is used at higher elevations. Conversely, if the device is designed for use at higher elevations, the device will tend to retract the needle significantly faster when the device is used at lower elevations. And if the device is designed for used at a medium elevation, the device will tend to retract the needle too fast at lower elevations and too slowly at higher elevations. Heretofore there has not been a needle retraction mechanism that makes use of a vacuum as the biasing mechanism that can ensure that the retraction speed of the needle remains within a narrow range of values even where the device is used at low and high elevations.